Linear transmitters are known. A Cartesian feedback transmitter constitutes one such known transmitter. When properly adjusted, such a negative feedback based transmitter architecture will operate in a satisfactory linear mode. At least two problems plague ordinary operation of such a transmitter, however.
First, loop phase shift must be properly adjusted; if not, transmitter stability will suffer. At worst, the negative feedback operation will function in a positive mode, causing undesirable oscillation.
Second, care must be taken to ensure that the final gain stages of the transmitter do not operate at or above their clip level. If clip levels are exceeded, the negative feedback operation of the transmitter will attempt to compensate, and the undesired results will include splatter in adjacent frequencies. This splatter can greatly disturb other radio operators.
One prior art approach has been to significantly limit baseband input values in order to ensure that clip levels will not be attained. Though this will succeed in avoiding the splatter problems noted above, the necessarily arbitrary limits placed on the baseband information are ordinarily so conservative as to prevent optimum operation of the transmission capabilities of the transmitter.
Accordingly, a need exists for a method and apparatus to provide for requisite training of a linear transmitter to assure proper loop phase adjustment and optimized baseband signal levels.